Heretofore, solvent extraction of hazardous, toxic, and radioactive wastes has been done with batch processes and with counter-current processes. Batch processes involved several leaching techniques, such as (i) heap-leach pile spraying, in which the solids were contacted with a uniform spray of solvent, which trickled through the pile, but did not immerse the pile; (ii) Fixed bed design, in which the solids were held in a cylindrical column and either immersed or sprayed with solvent, and the solvent was continuously removed from the top or bottom of the column; (iii) Agitated batch processing, in which solids were put in contact with a large volume of solvent by mechanical mixing.
Counter current processes also involved several leaching methods, including (iv) movement of the solids and liquids in opposite directions within a column; (v) movement of the solids and liquids in opposite directions within a fluidized bed using heavy linear agitation (such as mixers); and (vi) movement of liquids through solvents by using spinning vessels (such as centrifuges).
Each of these processes have their respective successes, but each also has a failure with regards to the removal of hazardous, toxic, and radioactive wastes from soils, sediments and debris. Of the batch methods, (i) heap leach pile spraying is very cost effective, but it is not effective in treating all of the solids. Sprays of solvent, even though uniformly applied will follow the path of least resistance, which are the most permeable paths. These paths will leach a majority of the pile, but will, however, leave areas that have been missed by the flow of solvent. These missed areas will be untreated. In the mining industry, these missed areas are of little economic value, and thus the overall process is a success. By contrast, in removing hazardous, toxic and radioactive wastes from soil, sediments, and debris, allowing some areas of solids to not Undergo treatment will pose an unreasonable risk to human health and the environment.
Batch process fixed bed designs (ii) can encounter the same problem of untreated solids areas as can heap-leach piles, if the solvent is sprayed on the solids, for the same reasons as detailed above. Of interest is that in immersion fixed bed methods, in which the solids are completely covered by the solvent, the same problem of untreated solids areas can also occur. When the solids are initially immersed in solvent, all solid particles are contacted with solvent (assuming that the process has been allowed sufficient time for the fluids to permeate to all portions of the solids area). Pumping of fluids through these particles or draining of fluids through these particles will create areas of preferred flow and other areas in which there is no flow. In the preferred flow areas, solvent that has dissolved the contaminants will be removed from the particles and replaced with cleaner fluids. In the no flow areas, the solvent that has dissolved the contaminants will stay in place. No amount of further pumping or draining of fluids in the immersed vessel will remove the contaminated solvent from the no flow area. Thus, when the solvent is eventually removed from the solids, the contaminated solvent will coat the particles in the immediate area of the drainage path from the "no flow" area, and will leave contamination in the soil, sediment, or debris.
Fixed bed batch processes also have a size, shape, and time limitation factor. The wider the cylindrical cavity, the higher the chance for channeling to develop. This can be solved by making the columns taller, but this has serious economic consequences, as the solids must be loaded into the columns, and transportation of the columns to the hazardous, toxic, or radioactive sites becomes an increasingly important economic factor in whether they will be used. The shape of fixed bed batch processes is limited due to "edge effects". In a container with comers, such as a rectangular vessel, the comers are not leached as efficiently as the middle of the vessel. This limitation is of profound economic significance, because of transportation considerations. Trucks can carry rectangular vessels with much greater ease and efficiency than can they transport cylinders. Time limitations come from the fact that if the solvents are pumped too fast through the vessels containing the solids, then channeling will increase greatly. The solvent has to be pumped through the solids slowly.
Agitated batch processing (iii) can eliminate the "no flow" areas that are not treated by other batch processes. This type of process has a profound economic problem in that the solvent ratio to solids processed is very high. The inventor has been witness to batch processes that require 5:1 solvent/solid ration for each cycle of each batch process. This large amount of solvent must either be disposed of properly or have the contaminants stripped out of the solvent before being reused in the process. This limits this type of process to solvents that can be economically disposed of in large quantities, or solvents that can be stripped very efficiently. Obviously, a process that would use less solvent for processing would be desirable, if for no other reason than the capital costs of the solvent alone.
Agitated batch processing gives a higher probability of all portions of the solids being treated, but the process involves more machinery and moving parts than heap-leach pile spraying, and fixed bed designs. The costs are therefore higher for capital costs, operational costs, and maintenance costs. Agitated batch processes also mobilize fine particles that may be present in the soils, sediments, or debris, which may cause additional costs to be spent to remove the fines from the solvent. At the other end of the particle size scale, agitated batch processes have great difficulty processing oversized materials commonly found at spill sites.
Counter current processes have a major advantage in that they typically use less solvent to perform the same amount of cleaning as do batch processes. Considerable amount of work has been done in trying to perfect these processes. The counter current process of (iv) movement of solids and liquids in opposite directions within a column has the disadvantages of immersed fixed bed batch processes described above. "No flow" zones still exist within these columns, leaving portions of the solids untreated, and the size, shape and time limitations described above are also a problem. In addition, the ability to reach a true steady state in which the solids and liquids are in counter-flow is very difficult in practice, and requires very specialized machinery with detailed process information. Slight changes in temperature, input feed, contamination levels, moisture contents, or other variables throws the system off, which may result in poorly treated solids areas. Further, this process has problems with oversized material, which tends to channel the solvent, leaving areas untreated.
Counter current processes using (v) fluidized beds and heavy linear agitation also have many of the same problems as described above for column counter current processes. They are not limited to cylindrical shaped extraction chambers, which is a point in their favor. They do require energy intensive mixing and again, extensive machinery and process control in order to work properly. Thus, again, costs are higher for capital costs, operational costs, and maintenance costs. Fines are also mobilized, which may cause additional costs to be spent to remove the fines from the solvent. Oversized material is also a problem. The energy intensive mixing is also loud.
Counter current devices and processes that employ (vi) spinning vessels have made progress in establishing the steady state required for counter current leaching, but are very complex in design and operation, with large numbers of moving parts. As with other counter current processes, they are limited by size, shape, and time constraints. Particle size of the input feed is also a problem with these systems, as oversized material can no be processed with these systems without further treatment (such as crushing). Again, similar to other complex machinery, they suffer from higher costs due to capital, operational, and maintenance costs.
As first mentioned above, solvent extraction processes for the removal of hazardous, toxic and radioactive wastes from soils, sediments, and debris have a special consideration over other systems used in the mining and other industries, in that untreated sections of solids is unacceptable to human health and the environment. In addition to this, solvent extraction processes used to remove hazardous, toxic and radioactive wastes must also compete in the marketplace against other technologies that deal with these wastes. Thus, economic considerations are a critical factor in determining applicability technology and forwarding the overall discipline. Current methods available that assure all untreated sections of solids are treated can use improvements that lower the economic capital costs, operational costs, maintenance costs, and transportation cost in moving the system to the contaminated site.